Austenitic alloy for heat strength with improved pouring and manufacturing, process for manufacturing billets and wire

ABSTRACT

Austenitic alloy for high-temperature strength with improved pourability and manufacturing, procedure for manufacturing billets and wire  
     Austenitic alloy for high-temperature strength with improved pourability and manufacturing, of which the composition comprises, in weight-%:  
     0.010%&lt;carbon&lt;0.04%  
     0%&lt;nitrogen&lt;0.01%  
     silicon&lt;2%  
     16%&lt;nickel&lt;19.9%  
     manganese&lt;8%  
     18.1%&lt;chromium&lt;21%  
     1.8%&lt;titanium&lt;3%  
     molybdenum&lt;3%  
     copper&lt;3%  
     aluminum&lt;1.5%  
     boron&lt;0.01%  
     vanadium&lt;2%  
     sulfur&lt;0.2%  
     phosphorous&lt;0.04%  
     and possibly up to 0.5% of at least one element chosen from among yttrium, cerium, lanthanum and other rare earths, the remainder being iron and impurities resulting from manufacturing or deoxidizing, the said composition also satisfying the two following relationships:  
     in relationship to the solidification mode:  
     remainder  
       a=eq. Ni a −0.5× eq. Cr a &lt;3.60  
     where  
       eq. Cr a =Cr+0.7×Si+0.2×Mn+1.37×Mo+3×Ti+6×Al+4×V,  
     and where  
       eq. Ni a =Ni+22×C+0.5×Cu,  
     in relationship to the rate of residual ferrite:  
     remainder  
       b=eq. Ni b −2× eq. Cr b &gt;−41  
     where  
       eq. Cr b =Cr+0.7×Si+1.37×Mo+3×Ti+6×Al+4×V,  
     and where  
       eq. Ni b =Ni+22×C+0.5×Cu+0.5×Mn.

[0001] The present invention concerns an austenitic alloy for heatstrength with improved pourability and manufacturing. The presentspecification incorporates by reference the complete disclosure of 0114818 filed Nov. 16, 2001.

[0002] Steels for high-temperature mechanical strength includemartensitic steels that can be used to around 550° C., non-oxidizingaustenitic steels containing a hardening intermetallic phaseprecipitation, which can be used to around 650° C. Alloys of nickel orcobalt are also used, generally hardened by intermetallic precipitation.

[0003] Non-oxidizing austenitic steels for high-temperature mechanicalstrength, such as the steel with reference no. 1.4980, according toEuropean standard EN 10269, also referenced as AlSi 660 according to thestandard ASTM A453, are frequently used in bolt and screw manufacturingand forged parts, in particular in fasteners for automotive exhaustelements, such as turbocompressors or exhaust pipes. They are alsofound, in the form of drawn wires, in mesh for mechanical trapping inexhaust gas catalytic converters. Applications for these steels are alsoknown in the area of springs that can be used at high temperature orexhaust hoses made up, on one hand, of rolled tubes—welded then crimped,and on the other hand, of metal wire mesh sheathing.

[0004] The composition of the steel AlSi 660 has a moderated chromiumcontent, on the order of 15%, about 1% molybdenum, 0.3% vanadium. Theaustenitic character, necessary for

[0005] high-temperature strength, is insured by a massive addition ofnickel, i.e., on the order of 24%.

[0006] The hardening and the resistance to creep are insured by anaddition of around 2% titanium, which is combined between 600° C. and750° C. with one part nickel to form intermetallics of the type Ni₃Ti.The steel composition can also contain elements such as Mo, V, Al whichalso contribute to hardening and high-temperature strength bysubstituting atoms of titanium in the Ni₃Ti compound.

[0007] The disadvantages of this steel are, in particular:

[0008] increased costs, particularly due to the significant nickelcontent,

[0009] difficulty in manufacturing since, at the time of pouring, thereare segregation formations which, unless specific precautions are taken,cause cracks in continuous pouring or at the time of hot rolling; as aresult, it is necessary to use a costly manufacturing process involvingremelting with grinding of the semi-finished products and increasedinspections of the finished products.

[0010] To reduce the segregations, the silicon must be limited to acontent of less than 0.3%, carbon to a content less than 0.050%, copperto a content lower than 0.5%, sulfur to a content less than 0.002%,phosphorous to a content less than 0.025%, lead to a content less than0.0005%, etc. These limitations represent the additional costs ofmanufacturing at the steel plant.

[0011] difficulty in rolling since the segregations greatly lower theburning point. Because of this, rolling must not be carried out abovearound 1150° C. in order to avoid the formation of serious defects, e.g.hot cracks. Taking into account the increased yield stress of the alloybelow this temperature, the rolling cannot be carried out except oncertain particularly robust systems. In addition, the rolling speed mustbe reduced in order to avoid any reheating above the burning point.

[0012] a limitation in the resistance to oxidizing and corrosion at hightemperature because of the low amounts of chromium and silicon, underparticularly intense exposure conditions, e.g. in exhaust lines.

[0013] difficulty in machining parts, particularly because of the smallamount of sulfur.

[0014] difficulty in welding, especially in the case of AlSi 660 sheetmetal welded to itself, with or without a supply of wire of the samealloy, since a great tendency to fissuring at high temperature isobserved.

[0015] In the family of austenitic steels for high-temperaturemechanical strength, hardened by intermetallic nickel-titaniumprecipitation, the following are known:

[0016] the steel AlSi 660 referenced above,

[0017] an IMPHY patent No. FR 94 14 942 that describes the followingcomposition:

[0018] Ni: 16% to 25%; Cr: 16% to 18.5%; Ti: >1%; Mn: 0% to 2%.

[0019] a NIPPON KOKAN patent JP 62267453 describing the followingcomposition: C<0.01%;

[0020] Ni: 10% to 18%; Cr: 13% to 20%; Ti: >1.5%; Mn: 0% to 2%.

[0021] Theoretical knowledge of the phases present at the time ofsolidification or in solid phase, in the quaternary alloys Fe—Cr—Ni—Tiremains incomplete. This was published by V. RAGHAVAN in 1996 in “Phasediagrams of quaternary iron alloys,” ed. The Indian Institute of Metals,pages 374 to 380. The range analyzed does not extend to compoundscontaining more than 1.7% Ti.

[0022] We have noted that the main difficulties encountered with thesteel AlSi 660 result from its solidification mode, which proves to bedirect solidification in austenitic form, in contrast to the majority ofnon-oxidizing austenitic steels, which solidify in ferrite, which thentransforms to austenite at lower temperature.

[0023] The alloy according to the IMPHY patent, with limited chromiumcontent, has austenitic solidification, as we will demonstrate in thefollowing. Thus it is subject to the problems in pouring and rollingthat are connected with segregations.

[0024] The composition of the alloy according to the NIPPON KOKAN patentshows a low amount of nickel mixed with a chromium content between 13%and 20%. The nickel content expresses itself inadequately to insurehardening and an effective creep resistance at 650° C. and above. Inaddition, the very small amount of carbon, less than 0.010% makes itunsuitable for manufacturing in air. In all cases, it probably does notsolidify to ferrite.

[0025] The goal of the invention is to propose an alloy of thenon-oxidizing austenitic type for high-temperature mechanical strength,which can be manufactured in an economical manner and is particularlyadapted to continuous pouring and to manufacturing at high temperature.

[0026] The object of the invention is an austenitic alloy forhigh-temperature strength with improved pourability and manufacturing,of which the composition is, in weight-%:

[0027] 0.010%<carbon<0.04%

[0028] 0%<nitrogen<0.01%

[0029] silicon<2%

[0030] 16%<nickel<19.9%

[0031] manganese<8%

[0032] 18.1%<chromium<21%

[0033] 1.8%<titanium<3%

[0034] molybdenum<3%

[0035] copper<3%

[0036] aluminum<1.5%

[0037] boron<0.01%

[0038] vanadium<2%

[0039] sulfur<0.2%

[0040] phosphorous<0.04%

[0041] and possibly up to 0.5% of at least one element chosen from amongyttrium, cerium, lanthanum and other rare earths, the remainder beingiron and impurities resulting from manufacturing or deoxidizing, thesaid composition also satisfying the two following relationships:

[0042] in relationship to the solidification mode:

[0043] remainder

a=eq.Ni_(a)−0.5×eq.Cr_(a)<3.60

[0044] where

eq.Cr_(a)=Cr+0.7×Si+0.2×Mn+1.37×Mo+3×Ti+6×Al+4×V,

[0045] and where

eq.Ni_(a)=Ni+22×C+0.5×Cu,

[0046] in relationship to the rate of residual ferrite:

[0047] remainder

b=eq.Ni_(b)−2×eq.Cr_(b)>−41

[0048] where

eq.Cr_(b)=Cr+0.7×Si+1.37×Mo+3×Ti+6×Al+4×V,

[0049] and where

eq.Ni_(b)=Ni+22×C+0.5×Cu+0.5×Mn.

[0050] In the preferred embodiments, the invention may contain thefollowing characteristics, taken alone or in combination:

[0051] the amount of chromium is greater than 18.5%,

[0052] the amount of manganese is greater than 2%,

[0053] the amount of silicon is greater than 1%,

[0054] the amount of nickel is greater than 18%,

[0055] the amount of aluminum is greater than 0.3%,

[0056] the amount of sulfur is greater than 0.030%,

[0057] the composition satisfies the following relationship, all amountsin mass-%:

[0058] in relationship to the absence of formation of the embrittlingsigma phase:

value c=Cr+1.5×Si+1.5×V+1.2Mo<22.

[0059] A second object of the invention is comprised of a manufacturingprocess for a billet of alloy of a composition conforming to theinvention and which includes the steps consisting of:

[0060] a) manufacturing the composition in air with electric furnace,

[0061] b) refining in A.O.D. converter,

[0062] c) continuous pouring in the form of blooms,

[0063] d) rolling the said blooms into billets at high temperature afterreheating to between 1100 and 1200° C.

[0064] A third object of the invention is made up by a fabricationprocess for alloy wire with composition conforming to the invention andwhich includes the steps consisting of:

[0065] e) hot rolling after reheating, to between 1100 and 1200° C., ofthe billets obtained by the process for manufacturing billets accordingto the invention, to obtain the wire rod,

[0066] f) annealing the said wire rod,

[0067] g) pickling it,

[0068] h) drawing or stretching it.

[0069] A fourth object of the invention is made up of a manufacturingprocess for bars of an alloy with composition conforming to theinvention and which includes the steps consisting of:

[0070] e) hot rolling, after reheating to between 1100 and 1200° C., ofthe billets obtained by the manufacturing process for billets accordingto the invention, to obtain bars

[0071] f) and annealing the said bars.

[0072] A fifth object of the invention is made up by alloy parts thatcan be obtained by machining or forming at low temperature or hightemperature, or processing, a wire or a bar obtained using one of theprocedures according to the invention, starting with a billet.

[0073] The description that follows and the figures attached, presentedin a non-limiting manner, will make the invention easy to understand.

[0074]FIGS. 1a and 1 b are micrographs in a state of roughsolidification showing the phases formed at the start of solidificationwith, on one hand in FIG. 1a, in an example of invention 13605, thepresence of ferrite with dendrite axis, clearly on the figure, and onthe other hand in FIG. 1b that corresponds to a counter-example thepresence of dendrites with austenitic axis in the IMPHY steel of theprior art.

[0075]FIGS. 2, 3 and 4 show the high-temperature ductility curves of thecompositions in table 1; the burning points, estimated by thetemperature at which the ductility is maximum, are given in Table 2. Theinvention presented concerns an austenitic alloy for high-temperaturestrength with improved pourability and manufacturing.

[0076] Following the studies carried out on pours, with determination ofthe solidification mode, the burning point, the phases present atequilibrium between 1060° C. and 1240° C., as well as at 720° C. and600° C., the ductility in traction at high temperature, called“forgeability,” the resilience at 20° C. and the resistance to creep at650° C., the inventors have found a general composition with which theproblems of steels and alloys presented in the prior art are resolved,in particular in the area of hardening, of resistance to creep, and mostespecially in the area of solidification, insuring a ferriticsolidification with a later transformation to solid phase of all of theferrite into austenite.

[0077] According to the invention, a composition 1 corresponds to thefollowing weighted composition:

[0078] Composition 1:

[0079] 0.010%<carbon<0.04%

[0080] 0%<nitrogen<0.01%

[0081] silicon<2%

[0082] 16%<nickel<19.9%

[0083] manganese<8%

[0084] 18.1%<chromium<21%

[0085] 1.8%<titanium<3%

[0086] molybdenum<3%

[0087] copper<3%

[0088] aluminum<1.5%

[0089] boron<0.01%

[0090] vanadium<2%

[0091] sulfur<0.2%

[0092] phosphorous<0.04%

[0093] and possibly up to 0.5% of at least one element chosen from amongyttrium, cerium, lanthanum and other rare earths, the remainder beingiron and impurities resulting from manufacturing or deoxidizing, thesaid composition also satisfying the two following relationships:

[0094] in relationship to the solidification mode:

[0095] remainder

a=eq.Ni_(a)−0.5×eq.Cr_(a)<3.60

[0096] where

eq.Cr_(a)=Cr+0.7×Si+0.2×Mn+1.37×Mo+3×Ti+6×Al+4×V,

[0097] and where

eq.Ni_(a)=Ni+22×C+0.5×Cu,

[0098] in relationship to the rate of residual ferrite:

[0099] remainder

b=eq.Ni_(b)−2×eq.Cr_(b)>−41

[0100] where

eq.Cr_(b)=Cr+0.7×Si+1.37×Mo+3×Ti+6×Al+4×V,

[0101] and where

eq.Ni_(b)=Ni+22×C+0.5×Cu+0.5×Mn

[0102] Composition 1 may also satisfy the following relationship:

[0103] in relationship to the absence of formation of the embrittlingsigma phase:

value c=Cr+1.5×Si+1.5×V+1.2Mo<22.

[0104] the hardening and the creep resistance are insured by theintermetallic precipitates with Ni₃Ti basis, obtained at the time ofaging treatments at around 700-750° C.,

[0105] the quantities of Ti and Ni are adequate to insure this hardeningprecipitation, the amount of nickel is definitely less than 24%,

[0106] the solidification mode is ferritic, surprisingly for this typeof alloy, with later transformation into solid phase of almost all ofthe ferrite into austenite,

[0107] the burning point, the temperature beyond which there is a lossin ductility in traction due to the start of local fusion is, in afavorable manner, greater than 1100° C. and preferably greater than1150° C.,

[0108] the weighted amount, value c, of sigmagenic elements Cr, V, Mo,Si is low enough to avoid the precipitation of the sigma phase andembrittlement at the time of use between 600 and 750° C.,

[0109] the combinations of forming elements for austenite or ferrite aredefined by the equivalents eqNi_(a) and eqCr_(a) as regards thesolidification mode and eqNi_(b) and eqCr_(b) as regards residualferrite after welding and annealing.

[0110] In addition, the composition satisfies the followingrelationships, all the elements in mass-%:

[0111] to insure the ferritic character of the solidification and anelevated burning point in relationship to the solidification mode:

[0112] remainder

a=eq.Ni_(a)−0.5×eq.Cr_(a)<3.60

[0113] where

eq.Cr_(a)=Cr+0.7×Si+0.2×Mn+1.37×Mo+3×Ti+6×Al+4×V,

[0114] and where

eq.Ni_(a)=Ni+22×C+0.5×Cu,

[0115] to limit, to trace amounts, the ferrite content aftermanufacturing at high temperature and annealing:

[0116] remainder

b=eq.Ni_(b)−2×eq.Cr_(b)>−41

[0117] where

eq.Cr_(b)=Cr+0.7×Si+1.37×Mo+3×Ti+6×Al+4×V,

[0118] and where

eq.Ni_(b)=Ni+22×C+0.5×Cu+0.5×Mn

[0119] and possibly:

[0120] to insure the absence of embrittlement at the time of use between600° C. and 750° C.:

Cr+1.5×Si+1.5×V+1.2×Mo<22

[0121] Preferably,

[0122] to improve creep resistance: Ni>18%

[0123] to improve resistance to oxidizing and the environment: Si>1%

[0124] to improve oxidizing and creep resistance: Al>0.3%

[0125] to improve machining capability: S>0.030%

[0126] In comparison with steels of the prior art mentioned, thefollowing can be noted:

[0127] an improvement in the resistance to oxidizing and corrosion athigh temperature,

[0128] an improvement in machining capability

[0129] the capability of welding the alloy according to the invention toitself in the scope of TIG or laser welding or as a supply material inthe scope of MIG or TIG welding with wire supply, with suppression ofthe tendency to high-temperature fissuring.

[0130] In another preferred composition 2 according to the invention.

[0131] Composition 2:

[0132]0.010%<carbon<0.04%

[0133] nitrogen<0.01%

[0134] 0.01%<silicon<2%

[0135] 16%<nickel<19.9%

[0136] 2%<manganese<8%

[0137] 18.1%<chromium<21%

[0138] 1.8%<titanium<3%

[0139] 0.01%<molybdenum<3%

[0140] 0.01%<copper<3%

[0141] 0.0005%<aluminum<1.5%

[0142] 0.0001%<boron<0.01%

[0143] 0.01%<vanadium<2%

[0144] 0%<sulfur<0.2%

[0145] phosphorous<0.04%, the rest being iron and other trace elements,residual elements or microadditions.

[0146] In addition, the composition satisfies the followingrelationships, with all the elements being in mass-%:

[0147] to insure the ferritic character of the solidification and theelevated burning point:

[0148] remainder

a=eq.Ni_(a)−0.5×eq.Cr_(a)<3.60

[0149] where

eq.Cr_(a)=Cr+0.7×Si+0.2×Mn+1.37×Mo+3×Ti+6×Al+4×V,

[0150] and where

eq.Ni_(a)=Ni+22×C+0.5×Cu,

[0151] to limit the amount of ferrite to traces after manufacturing athigh temperature and annealing:

[0152] remainder

b=eq.Ni_(b)−2×eq.Cr_(b)>−41

[0153] where

eq.Cr_(b)=Cr+0.7×Si+1.37×Mo+3×Ti+6×Al+4×V,

[0154] and where

eq.Ni_(b)=Ni+22×C+0.5×Cu+0.5×Mn

[0155] and possibly:

[0156] to insure the absence of embrittlement during use between 600 and750° C.:

Cr+1.5×Si+1.5×V+1.2×Mo<22.

[0157] Preferably:

[0158] to improve creep resistance: Ni>18%

[0159] to improve resistance to oxidizing and the environment: Si>1%

[0160] to improve oxidizing and creep resistance: Al>0.3%

[0161] to improve machining capability: S>0.030%

[0162] In composition 2 of the invention, the manganese content isgreater than 2%.

[0163] According to the invention, the relationships make it possible toselect ferritic solidification compositions, without residual ferriteand do not form sigma phase.

[0164] Table 1 presents examples of pours carried out in a vacuum toachieve the alloy according to the invention, as well ascounter-examples of pours that do not correspond to the invention andcompositions according to the prior art cited.

[0165] The following in particular were studied:

[0166] a) on the ingot:

[0167] the solidification mode, by micrography,

[0168] the ferrite quantity measured by magnetic method on the roughingot and the ingot reheated 15 min. to 1240° C.

[0169] b) on the product finished by forging:

[0170] the quantities of residual ferrite, by magnetic measurements,after annealing 1 hour at 980° C. or 1060° C.,

[0171] the tensile ductility in high-temperature with an increase in thetest temperature to a speed of 10° C./s, maintaining it for 80 s, arapid traction at 14 s-1, measurement of the reduction in diameter. Fora series of tests at increasing temperature, the temperature isevaluated starting from that of the ductility dropping rapidly, as aresult of the start of local fusion. This temperature, called theburning point, must not by exceeded in reheating before rolling andduring rolling, at the risk of creating defects.

[0172] c) on the product finished by forging, then annealed for one hourat a temperature of 980° C. or 1060° C., then aged for 16 hours at atemperature of 720° C.:

[0173] the presence of the sigma phase by micrography and, when there isany, of the quantity of the sigma phase by an X-ray diffraction method,

[0174] the mechanical properties in traction, the strength andresistance at ambient temperature,

[0175] resilience after additional aging of 200 h at 600° C.,

[0176] resistance to creep to break at 650° under 385 Mpa by measuringthe time at break and elongation at break.

[0177] In the scope of solidification of the alloy, for the compositionaccording to the invention, the solidification takes place in the formof ferritic dendritic axes, which contain the residual ferrite aftercooling, as shown in FIG. 1a, in contrast to the known and observedcases of steel with reference AlSi 660 and the alloy according to theIMPHY patent, of which the solidification starts with the formation ofaustenite, as shown in FIG. 1b.

[0178] It has been possible to establish that, in the scope of thecomposition according to the invention, the criterion:

[0179] remainder

a=eq.Ni_(a)−0.5×eq.Cr_(a)<3.60

[0180] where

eq.Cr_(a)=Cr+0.7×Si+0.2×Mn+1.37×Mo+3×Ti+6×Al+4×V,

[0181] and where

eq.Ni_(a)=Ni+22×C+0.5×Cu,

[0182] makes it possible to select the compositions with ferriticsolidification.

[0183] The presence of more than 1% ferrite after reheating to 1240° C.also translates into the possibility of existence of this phase withequilibrium at high temperature, near the solidification point.

[0184]FIGS. 2, 3 and 4 show the high-temperature ductility curves forthe compositions studied; the ductility is measured by delta Ø, which isthe reduction in diameter at break, i.e., the relative variation indiameter at the level of the break; the burning points estimated usingthe temperature at which ductility is maximum are shown in Table 2.

[0185] It appears that the solidification in ferritic mode or startingwith ferrite makes it possible to obtain the burning points greater than1100° C., in contrast to solidification in austenitic mode.

[0186] Solidification in ferritic mode, obtained when the criterionabove is complied with, makes it possible to reheat and roll the ingotsor semi-finished products at normal speed between 1100 and 1200° C.,preferably between 1120 and 1180° C., within a range of normaltemperatures for non-oxidizing steels and compatible with the reheatingfurnaces and the mechanical dimensions of the rollers.

[0187] The residual ferrite measured in the product finished by forgingfrom 1100° C. into an 18-mm octagonal bar and annealed 1 hour at 980° C.or 1060° C. is indicated in Table 2.

[0188] Certain compositions with ferritic solidification contain morethan 1% ferrite. This residual ferrite should have a resistance to creepthat is less than that of the austenitic phase.

[0189] The criterion:

[0190] remainder

[0191]b=eq.Ni_(b)−2×eq.Cr_(b)>−41

[0192] where

eq.Cr_(b)=Cr+0.7×Si+1.37×Mo+3×Ti+6×Al+4×V,

[0193] and where

eq.Ni_(b)=Ni+22×C+0.5×Cu+0.5×Mn

[0194] makes it possible to select compositions with ferriticsolidification that have less than 3% residual ferrite after process inthe range 980° C.-1060° C., in such a way as to limit the loss of creepresistance.

[0195] After welding by forging, annealed at 980° C. or 1060° C. and 16hours aging at 720° C., all the compositions were observed using opticalmetallography after electro-nitric attack. In addition to the residualferrite, in certain compositions, the presence of an intermetallic phaseis observed, which has been identified by X-ray diffraction as being thesigma phase. The quantitative measurements are indicated in Table 2.

[0196] The presence of the sigma phase is known to decrease theresilience and the strength of austenitic steels. A criterion has beendetermined that makes it possible to insure the absence of the sigmaphase in the aged state:

Cr+1.5×Si+1.5×V+1.2×Mo<22.

[0197] The criterion above thus makes it possible to insure a resiliencelevel that is adequate in the processed state, as well as after usage athigh temperature.

[0198] Table 2 indicates the traction characteristics and the strengthmeasured at ambient temperature after forging, annealing of 1 hour at980° C. or 1060° C. and aging for 16 hours at 720° C.

[0199] The elevated hardnesses are obtained for melts 13606 and 13604,due to the formation of the sigma phase.

[0200] The characteristics obtained for melts 13747, 13748 and 13605 areclose to those of the cells of grade AlSi 660.

[0201] Pour 13470, with greater amounts of Ni and Ti, presents moreimproved characteristics.

[0202] The creep tests to break at 650° C. at 385 MPa have been carriedout on the pours 13468 Imphy and 13605. The requirements usually set formounting at high temperature, in particular greater than 100 hours atbreak, and greater than 5% extension at break are complied with.

[0203] According to the invention, a minimum carbon content of 0.010% isnecessary to allow manufacturing “in air” in the systems such aselectric furnace plus AOD refining and in the ladle without using vacuumor low pressure.

[0204] A maximum carbon content of 0.040% is necessary to avoid greatlylowering the liquidus of the alloy and increasing the solidificationinterval of the alloy, making continuous pouring impossible.

[0205] In addition, the carbon combines with part of the titanium in theform of TiC type carbides which is no longer available for strengtheningthe alloy in the form of Ni₃Ti in the aged state. It is necessary tominimize this phenomenon by limiting the carbon content.

[0206] A maximum nitrogen content of 0.010% is the result of thereaction, in the liquid metal, of the titanium added in large quantitywith the nitrogen that is already present: there is a formation anddecantation of the TiN nitrides in the ladles and the pour distributorsand the nitrogen content of the poured product must not exceed thepreceding value.

[0207] Silicon is generally present in the composition, at least intrace amounts of which the level is 0.001% in the steel products.

[0208] Silicon contributes to the formation of ferrite and sigma phase.A maximum content of 2.0% is necessary to avoid accelerated formation ofthis latter embrittling phase.

[0209] The silicon contributes to improvement in resistance to oxidizingand the environment at high temperature, by forming more or lesscontinuous layers of silica or silicates under the other oxides. Asignificant addition, e.g. of more than 1%, is thus useful when thesolidification occurs in ferritic mode. A notable addition, e.g. between0.2 and 2%, is possible without formation of significant segregations,as may be the case in certain solidification processes when thesolidification occurs in austenitic mode.

[0210] A minimum manganese content of 0.001% is generally present as aresidue deriving especially from the ferroalloys.

[0211] At the time of manufacturing, the manganese oxidizes easilyduring oxygen blasts intended to bring the carbon to the level required;a maximum content of 8% is necessary to permit refining under correctproduction conditions with the addition of manganese.

[0212] We have found that the manganese presents the specific feature ofpromoting the ferritic solidification mode, while promoting, incontrast, the suppression of the residual ferrite at the time ofannealing between 900° C. and 1200° C., notably on the productmanufactured at high temperature. It does not cause the formation ofsigma phase.

[0213] Since it is necessary to obtain the ferritic solidification modewhile avoiding an excess of other elements that form ferrite, such asCr, Mo, Si, W, an excess which would cause embrittlement by forming thesigma phase at the time of aging, manganese proves to be especiallyuseful when the goal is to greatly harden the alloy using a significantnickel content.

[0214] The addition of manganese causes an increase in the thickness ofscales on products rolled at high temperature or annealed or at the timeof use. A silicon addition, e.g. of more than 1%, then makes it possibleto bring the oxidizing back to a normal level.

[0215] A minimum nickel content of 16%, in combination with titaniumcontent greater than 1.8%, is necessary to obtain a significanthardening at the time of aging between 650° C. and 750° C. Thishardening by precipitation of intermetallics, of the type Ni₃Ti, isnecessary for the mechanical strength at ambient temperature offastenings, as well as for their resistance to tension and creep at hightemperature.

[0216] A maximum nickel content of 19.9% is imposed, particularly foreconomic reasons.

[0217] To improve the creep resistance, it is possible to add nickelabove 18%. Under these conditions, the hardening that is produced at thetime of aging at 720° C. practically reaches its maximum.

[0218] Taking into account the level of nickel necessary to harden thealloy, a minimum chromium content of 18.1% is necessary to balance theeffect of austenite formation from the nickel and to obtain ferriticsolidification, especially when the other elements that form ferrite,such as Si, Mo, Mn, Ti, Al, V are at a low level or close to theirminimum amounts.

[0219] A maximum chromium content limited to 21% is necessary to avoidthe formation of the embrittling sigma phase at the time of processingat 720° C. or use in the range between 600° C. and 700° C.

[0220] A minimum titanium content of 1.8% is necessary to obtainadequate hardening at the time of aging treatments or at the time of usein the range between 600° C. and 750° C. A fine precipitation with Ni₃Tibasis then forms which contributes to the high-temperature mechanicalstrength, especially in creep conditions.

[0221] Titanium is also present in the alloy in the form of titaniumnitride, titanium carbide and titanium phosphide.

[0222] A content limited to 3.0% is necessary to avoid lowering theliquidus and the formation, at the time of solidification, of largeintermetallics that could impair drawing capability.

[0223] A minimum molybdenum content of 0.010% is generally present intraces at the time of industrial production.

[0224] The molybdenum contributes to the formation of ferrite at thetime of solidification and to the formation of hardening intermetallics,by substituting titanium atoms. The addition of molybdenum makespossible an improvement in the high-temperature strength of the alloy,thus increasing the content of precipitates and the shearing resistance.

[0225] A maximum content of 3% is necessary to prevent the formation ofthe sigma phase in connection with the chromium, as well as the presenceof residual ferrite.

[0226] A minimum copper content of 0.010% is generally present in theform of manufacturing residue.

[0227] The copper contributes to the formation of austenite and makes itpossible to reduce the rate of residual ferrite, in the same way thatnickel does.

[0228] A maximum content of 3% is imposed to prevent great segregationsat the time of pouring and the formation of a phase that is rich incopper that greatly lowers the burning point.

[0229] A minimum content of 0.0005% aluminum is generally present in theform of manufacturing residue.

[0230] The addition of aluminum makes it possible to increase thecontent of hardening precipitates and the high-temperature strength bysubstituting titanium atoms.

[0231] In addition, the aluminum can be used to increase the ferriticcharacter of the alloy at the time of solidification without having thedisadvantage of generating the embrittling sigma phase when maintainedat temperatures in the range between 550° C. and 700° C.

[0232] A maximum aluminum content of 1.5% is necessary to avoidexhaustion of the nickel at the time of intermetallic formation and thepresence of residual ferrite.

[0233] A minimum boron content of 0.0001% is generally present in theform of trace amounts.

[0234] The presence of boron in amounts of 10 to 30 ppm, for example,allows a slight improvement in the high-temperature ductility in thetemperature range between 800° C. and 1100° C.

[0235] A maximum content of 0.01% is necessary to prevent excessivelowering of the solidus and of the burning point that it causes.

[0236] A minimum vanadium content of 0.01% is generally present in theform of manufacturing residue.

[0237] The vanadium, the ferritizing element and former of the sigmaphase, may be added to contribute to the hardening by substitution ofthe titanium atoms in the intermetallic compounds.

[0238] A maximum vanadium content of 2% is necessary to prevent theformation of the sigma phase, in combination with the chromium present.

[0239] A minimum sulfur content of 0.0001% is generally present as arefining residue.

[0240] The sulfur can be maintained deliberately, or added at preferablymore than 0.030% to improve the machining capability of the alloy due tothe presence of titanium sulfides and carbosulfides formed at the timeof solidification which improve the fragmentation of chips. Thisaddition is made possible by the ferritic solidification mode, since theaddition of sulfur does not greatly decrease the high-temperatureductility at the time of rolling, in contrast to the prior art, withaustenitic solidification and pronounced segregations.

[0241] A maximum content of 0.2% is necessary to prevent the risks oflongitudinal opening of the semi-finished products, along the elongatedsulfides at the time of high-temperature rolling.

[0242] A minimum phosphorous content of 0.001% is generally present inthe form of manufacturing residue.

[0243] A maximum phosphorous content of 0.040% is necessary to preventthe presence of large particles of titanium phosphides formed at thetime of solidification and that can impair drawing capability.

[0244] Other elements, such as cobalt, tungsten, niobium, zirconium,tantalum, hafnium, oxygen, magnesium, calcium may be present in the formof manufacturing or deoxidizing residues; other elements may be addeddeliberately in quantities that do not exceed 0.5% to improve specificproperties such as oxidizing resistance by microaddition of yttrium,cerium, lanthanum and other rare earths.

[0245] An example of industrial use of a steel according to theinvention and the properties of the final industrial product accordingto the invention:

[0246] On industrial production tools, a pour of 35 tons, no. 141067 wascarried out with the composition according to the invention, indicatedin Table 1. The operations carried out, successfully and with a low rateof defects with this pour, were as follows:

[0247] a) manufacture in air in electric furnace

[0248] b) refining in A.O.D. converter

[0249] c) continuous pouring in the form of blooms of 1 ton with squaresection 205×205 mm

[0250] d) hot rolling at around 1100° C. in 500 kg billets with squaresection of 120×120 mm

[0251] e) hot rolling of the 120×120 mm billets at around 1100° C. incoils of 500 kg with 5.5 mm wire rod

[0252] f) annealing in coils

[0253] g) pickling

[0254] h) drawing

[0255] In comparison, the same operations were carried out on severalpours of the grade AlSi 660, which gave rise to numerous defects (crackson blooms, fissures on billets, flaws and scale on wire rod). Usually,the grade AlSi 660 is poured in the form of ingots without using thecontinuous pouring process.

[0256] As a result, this industrial test has demonstrated thesuperiority of the composition according to the invention for obtainingwire rods of non-oxidizing steel with high-temperature strength by aneconomical process including manufacturing in air, AOD refining and acontinuous bloom pouring process.

[0257] In comparison to the non-oxidizing austenitic steels forhigh-temperature fasteners of the prior art, the alloy according to theinvention presents several advantages:

[0258] a) ease in pouring, with ferritic solidification mode making itpossible to pour blooms or slabs in continuous process without formationof pouring defects, central segregations, segregated wires, hotcracking; thus the need to pour in ingot followed by a supplementaryblooming or stabbing operation is prevented, which is necessary for thealloys of the prior art.

[0259] b) cost-effectiveness in raw materials, especially nickel, incomparison to AlSi 660 steels currently in use.

[0260] c) ease in manufacturing; in fact, in contrast to alloys of theprior art, it is not necessary to try to obtain especially low contentsof silicon, copper, sulfur, phosphorous, lead, antimony, bismuth toprevent the problems of segregation and hot fissuring and segregations;as a result, the raw material batch is simpler and more economical, andmanufacturing “in air” in electric furnace and AOD, without passagethrough vacuum or low pressure becomes possible.

[0261] d) ease in rolling, reheating and rolling of ingots, blooms fromcontinuous pouring and semi-finished products is possible between 1100°C. and 1200° C.; for the alloys of the prior art it is not possible,without risk of hot cracks and fissuring, to go above 1100° C. on therough products of pouring and 1150° C. after a first rolling.

[0262] As a result, the installations dimensioned for currentnon-oxidizing steels can be used to roll this steel, and it is notnecessary to greatly decrease the rolling speed to prevent internalfissuring by overheating at the end of the rolling.

[0263] e) resistance to oxidizing and to the environment. The alloyproposed advantageously contains a high chromium content, which insuresgood resistance to oxidizing and to corrosion at high temperature at thetime of use, e.g. between 500° C. and 750° C. In addition, it maycontain silicon, which plays the same role.

[0264] f) improved machining capability if sulfur is added, e.g. greaterthan 0.030%, which makes it possible to restore proper machiningcapability, which is the opposite of the AlSi 660 steel and the otheralloys of the prior art that do not contain sulfur, since theirprocessing at high temperature becomes impossible if the sulfur ispresent in significant quantity.

[0265] g) good welding capability; the alloy proposed can be welded witha very reduced tendency to high-temperature fissuring in comparison toalloys of the prior art, due to its ferritic solidification mode and theabsence of large solidification segregations. In particular, it can bewelded to itself using TIG or laser or by electric resistance welding,or be used as metal supply wire for MIG or TIG or in electrodes forwelding.

[0266] The alloy according to the invention can be used, in particular,in the following applications:

[0267] internal furnace fittings,

[0268] parts for cement furnaces,

[0269] inlet or exhaust valves for automotive engines,

[0270] fasteners and bolts and screws for automotive exhaust systems,

[0271] springs used at high temperature,

[0272] braids of wire and tubular walls for corrugated tubes for, e.g.automotive exhaust systems,

[0273] wire mesh for e.g. furnace transporting mats, mechanical trappingfor exhaust catalytic converters,

[0274] fibers and fiber mesh for presses used for hot forming of glass,

[0275] welded sheets, e.g. for turbine combustion chambers,

[0276] welding support wire, machined bars,

[0277] turbine synchro ring with blades fastened in variable orientationfor automotive turbocompressors, sheet metal parts,

[0278] annular sealing segments for automotive turbocompressors. TABLE 1Counter- Compositions according to Examples examples the prior artPour/grade Nippon Imphy C N Si Mn Ni Cr Ti Mo Cu Al B V S P eq. Cr_(a)eq. Ni_(a) Remainder a eq. Cr_(b) eq. Ni_(b) Remainder b value c

[0279] TABLEAU 1 Exemples Coulé/nuance 13748 13747 13605 13794 14106713883 13822 13824 C 0.022 0.022 0.022 0.020 0.021 0.018 0.020 0.020 N0.009 0.007 0.005 0.008 0.004 0.006 0.006 0.006 Si 0.251 0.503 0.4540.245 0.186 0.261 1.300 0.250 Mn 0.488 6.233 0.420 0.342 1.760 1.7954.000 1.700 Ni 17.13 17.19 16.26 17.35 17.34 17.16 18.20 18.00 Cr 19.2418.57 19.10 19.14 19.54 18.57 18.90 19.00 Ti 2.078 2.018 1.950 2.2142.074 2.146 2.150 2.500 Mo 1.261 1.262 1.236 1.240 1.270 1.244 0.8001.250 Cu 0.201 0.104 0.100 0.205 0.064 0.204 0.200 0.200 Al 0.155 0.1610.195 0.157 0.165 0.180 0.350 0.175 B 0.0018 0.0015 0.0013 0.0013 0.00210.0013 0.0012 0.0012 V 0.075 0.075 0.077 0.075 0.156 0.101 0.080 0.100 S0.0006 0.0007 0.0022 0.0034 0.0011 0.0023 0.0005 0.1000 P 0.018 0.0110.014 0.015 0.014 0.019 0.015 0.015 éq Cr_(a) 28.70 29.22 28.52 28.9629.60 28.74 30.58 30.18 éq Ni_(a) 17.71 17.73 16.79 17.89 17.83 17.6618.74 18.54 reliquat a 3.36 3.12 2.53 3.41 3.03 3.29 3.45 3.45 éq Cr_(b)28.61 27.97 28.44 28.89 29.25 28.38 29.78 29.84 éq Ni_(b) 17.96 20.8417.00 18.06 18.71 18.56 20.74 19.39 reliquat b −39.3 −35.1 −39.9 −39.7−39.8 −38.2 −38.8 −40.3 valeur c 21.2 21.0 21.4 21.1 21.6 20.6 21.9 21.0Compositions selon Contre-exemples l'art antérieur Coulé/nuance 1347013606 113604 1.4980 Nippon Imphy C 0.017 0.021 0.021 0.041 0.004 0.033 N0.006 0.008 0.011 0.004 0.006 0.006 Si 0.623 0.470 0.490 0.103 0.8500.476 Mn 0.391 0.400 0.405 1.827 1.480 0.970 Ni 18.36 18.26 16.05 24.8317.50 18.42 Cr 19.08 21.18 21.20 14.67 19.60 17.17 Ti 2.366 1.912 2.0942.158 1.830 2.250 Mo 1.253 1.242 1.241 1.249 0.000 1.240 Cu 0.101 0.0990.100 0.108 0.098 Al 0.164 0.171 0.150 0.162 0.203 B 0.0013 0.00140.0013 0.0042 0.0014 V 0.073 0.073 0.075 0.327 0.068 S 0.0017 0.00230.0018 0.0010 0.0010 P 0.028 0.014 0.013 0.019 0.009 éq Cr_(a) 29.6830.34 30.81 25.57 25.98 27.64 éq Ni_(a) 18.78 18.77 16.56 25.79 17.5919.20 reliquat a 3.94 3.60 1.16 13.00 4.60 5.38 éq Cr_(b) 29.61 30.2630.73 25.21 25.69 27.44 éq Ni_(b) 18.98 18.97 16.76 26.70 18.33 19.68reliquat b −40.2 −41.6 −44.7 −23.7 −33.0 −35.2 valeur c 21.6 23.5 23.516.8 20.9 19.5

[0280] TABLE 2 Compositions Examples Counter-examples according to priorart Pour/grade Imphy Solidification micrographic observations ferrite %measurement on rough ingot (%) ferrite 1240° C. measurement on ingotprocessed 15 min. at 1240° C. (%) burning point tests ofhigh-temperature traction (° C.) ferrite 980° C. measurement in finishedstate, annealed 1 h at 980° C. ferrite 1060° C. measurement in finishedstate, annealed 1 h at 1060° C. sigma 720° C. measurement/finishedstate, processed 1 h at 980° C. + 16 h at 720° C. sigma 720° C.measurement/finished state, processed 1 h at 1060° C. + 16 h at 720° C.Hardness at 20° C. in aged state 16 h at 720° C. (Hv 5 kg) Rm (Mpa) at20° C. in aged state 16 h at 720° C. E0.2 (Mpa) at 20° C. in aged state16 h at 720° C. A % at 20° C. in aged state 16 h at 720° C. Resilienceat 20° C. in aged state 16 h at 720° C. (dal/cm²) Resilience at 20° C.for 16 h at 720° C. + 200 h at (dal/cm²) 600° C. Rm(Mpa) at 20° C. for16 h at 720° C. + 200 h at 600° C. creep to break (h) at 650° C. at 385MPa for 16 h at 720° C. creep to break (A %) at 650° C. at 385 MPa for16 h at 720° C.

[0281] TABLEAU 2 Exemples Coulée/nuance 13748 13747 13605 13794 14106713883 solidification observation micrographique F F F F F F ferrite %mesure sur lingot brut (%) 1.60 0.70 5.40 1.40 0.80 1.00 ferrite 1240°C. mesure sur lingot traité 15 min à 11.00 8.00 11.00 4.10 8.40 4.201240° C. (%) point de brûlure (° C.) essais de traction à chaud 11501150 1180 1150 ferrite 980° C. mesure sur état corroyé recuit 1 h 0.440.42 0.50 0.70 à 980° C. ferrite 1060° C. mesure sur état corroyé recuit1 h 2.10 0.40 à 1060° C. sigma 720° C. mesure/corroyé traité 1 h à 0.000.00 980° C. + 16 h à 72° C. sigma 720° C. mesure/corroyé traité 1 h à0.00 0.00 1060° C. + 16 h à 720° C. dureté (Hv 5 kg) à 20° C. sur étatvieilli 16 h à 720° C. 340 285 291 310 243 Rm (Mpa) à 20° C. sur étatvieilli 16 h à 720° C. 963 850 869 E0.2 (Mpa) à 20° C. sur état vieilli16 h à 720° C. 615 492 511 A % à 20° C. sur état vieilli 16 h à 720° C.19 16 31 résilience(daJ/cm²) à 20° C. sur état vieilli 16 h à 720° C.10.7 résilience(daJ/cm²) à 20° C. sur 16 h à 720° C. + 7.7 200 h à 600°C. Rm (Mpa) à 20° C. sur 16 h à 720° C. + 200 h à 1128 1015 1060 600° C.fluage rupture (h) à 650° C. sous 385 MPa sur 16 h à 124 167 720° C.fluage rupture (A %) à 650° C. sous 385 MPa sur 16 h à 16.0 7.4 720° C.Compositions selon Contre-exemples l'art antérieur Coulée/nuance 1347013606 13604 1.4980 Imphy solidification observation micrographique A + FF + A F A A ferrite % mesure sur lingot brut (%) 0.70 1.10 2.50 0.40ferrite 1240° C. mesure sur lingot traité 15 min à 2.04 11.00 33.00 0.431240° C. (%) point de brûlure (° C.) essais de traction à chaud 11001150 1140 1080 1100 ferrite 980° C. mesure sur état corroyé recuit 1 h0.40 0.50 0.60 0.00 0.40 à 980° C. ferrite 1060° C. mesure sur étatcorroyé recuit 1 h 0.90 5.50 à 1060° C. sigma 720° C. mesure/corroyétraité 1 h à 0.00 980° C. + 16 h à 72° C. sigma 720° C. mesure/corroyétraité 1 h à 0.00 4.50 12.00 0.00 1060° C. + 16 h à 720° C. dureté (Hv 5kg) à 20° C. sur état vieilli 16 h à 720° C. 320 339 401 333 310 Rm(Mpa) à 20° C. sur état vieilli 16 h à 720° C. 1034 988 1019 1046 952E0.2 (Mpa) à 20° C. sur état vieilli 16 h à 720° C. 650 663 765 699 589A % à 20° C. sur état vieilli 16 h à 720° C. 31 25 16 25 26résilience(daJ/cm²) à 20° C. sur état vieilli 16 h à 720° C. 2.1 10.910.9 résilience(daJ/cm²) à 20° C. sur 16 h à 720° C. + 1.6 9.1 9.1 200 hà 600° C. Rm (Mpa) à 20° C. sur 16 h à 720° C. + 200 h à 1133 600° C.fluage rupture (h) à 650° C. sous 385 MPa sur 16 h à 300 250 720° C.fluage rupture (A %) à 650° C. sous 385 MPa sur 16 h à 11.0 6.1 720° C.

1. Austenitic alloy for high-temperature strength with improvedpourability and manufacturing, of which the composition comprises, inweight-%: 0.010%<carbon<0.04% 0%<nitrogen<0.01% silicon<2%16%<nickel<19.9% manganese<8% 18.1%<chromium<21% 1.8%<titanium<3%molybdenum<3% copper<3% aluminum<1.5% boron<0.01% vanadium<2%sulfur<0.2% phosphorous<0.04% and possibly up to 0.5% of at least oneelement chosen from among yttrium, cerium, lanthanum and other rareearths, the remainder being iron and impurities resulting frommanufacturing or deoxidizing, the said composition also satisfying thetwo following relationships: in relationship to the solidification mode:remainder a=eq.Ni_(a)−0.5×eq.Cr_(a)<3.60 whereeq.Cr_(a)=Cr+0.7×Si+0.2×Mn+1.37×Mo+3×Ti+6×Al+4×V, and whereeq.Ni_(a)=Ni+22×C+0.5×Cu, in relationship to the rate of residualferrite: remainder b=eq.Ni_(b)−2×eq.Cr_(b)>−41 whereeq.Cr_(b)=Cr+0.7×Si+1.37×Mo+3×Ti+6×Al+4×V, and whereeq.Ni_(b)=Ni+22×C+0.5×Cu+0.5×Mn
 2. Alloy according to claim 1, alsocharacterized in that the chromium content is greater than 18.5%. 3.Alloy according to any one of claims 1 or 2, also characterized in thatthe manganese content is greater than 2%.
 4. Alloy according to any oneof claims 1 to 3, also characterized in that the silicon content isgreater than 1%.
 5. Alloy according to any one of claims 1 to 4, alsocharacterized in that the nickel content is greater than 18%.
 6. Alloyaccording to any one of claims 1 to 5, also characterized in that thealuminum content is greater than 0.3%.
 7. Alloy according to any one ofclaims 1 to 6, also characterized in that the sulfur content is greaterthan 0.030%.
 8. Alloy according to any one of claims 1 to 7, alsocharacterized in that the composition satisfies the followingrelationship: in relationship to the absence of formation of theembrittling sigma phase: value c=Cr+1.5×Si+1.5×V+1.2 Mo<22.
 9. Processfor manufacturing a billet of alloy, of which the composition isaccording to any one of claims 1 to 8, characterized in that it includesthe steps consisting of: a) manufacturing the composition in air withelectric furnace, b) refining with A.O.D. converter, c) continuouspouring in the form of blooms, d) rolling the said blooms into billetsat high temperature with reheating between 1100 and 1200° C.
 10. Processfor manufacturing wire of alloy, of which the composition is accordingto any one of claims 1 to 8, characterized in that it includes the stepsconsisting of: e) hot rolling after reheating, between 1100 and 1200°C., of the billets obtained by the process according to claim 9, toobtain the wire rod, f) annealing the said wire rod, g) pickling it, h)drawing or stretching it.
 11. Process for manufacturing bars of alloy,of which the composition is according to any one of claims 1 to 8,characterized in that it includes the steps consisting of: e) hotrolling, after reheating between 1100 and 1200° C., of the billetsobtained by the process according to claim 9, to obtain bars, f) andannealing the said bars.
 12. Alloy part that can be obtained by hot orcold machining or forming, or meshing—starting with a billet—a wire or abar obtained by the process according to any one of claims 9 to 11.